It is now recognised emissions from marine vessels or ships in inland waterways and coastal areas can be a significant contribution to air quality. Marine engines can be electric engines, spark ignition engines, compression ignition engines or turbines. Ships in the context of the present invention are to include all floating objects.
In order to reduce NOX emissions of marine engines, NOX reducing exhaust gas treatment systems have been developed. These NOX reducing systems typically comprise a NOX reducing catalyst.
In U.S. Pat. No. 5,412,946 for example, a NOX reducing catalyst comprising Pt on zeolite have been described. Such a catalyst promote the reduction of NOX to nitrogen in the presence of a reducing compound. It has been described in the art to use hydrocarbons, hydrogen or synthesis gas as reducing compound for this type NOX reducing catalyst.
NOX reducing systems that comprise both a deNOX catalyst and a NOX sorbent are also known in the art. For example from U.S. Pat. No. 5,874,057, U.S. Pat. No. 5,473,887 and WO 01/34950. During lean operation, NOX is absorbed from the exhaust gas; during richer operation, the sorbent is regenerated and the catalyst promotes reduction of NOX to nitrogen. It is disclosed that the exhaust gas can periodically be made richer (less oxygen) by adding fuel, hydrogen or synthesis gas to the lean exhaust gas.
An alternative method to reduce emissions of compression ignition internal combustion engines is by means of a process known in the art as fumigation. In a fumigation process, a gaseous fuel is mixed with the intake air of the engine prior to introducing the air/gaseous fuel mixture into the engine cylinder. Both diesel fuel and the air/gaseous fuel mixture are introduced into the engine. Known gaseous fuels for fumigation are for example natural gas, liquefied petroleum gas (LPG), and hydrogen gas.
Another method for reducing emissions, especially NOX, in an internal combustion engine is by exhaust gas recirculation (EGR). NOX emissions are reduced with the increase of exhaust gas recirculation. A high level of recirculation, however, can result in poor combustion. Various ways have been reported in the art by which recirculated exhaust gas may be enriched. In L. K. S Teo et al. “Hydrogen and Biodiesel Mixtures as Fuels for the Compression Ignition Engine” Proceedings of the THIESEL 2002 Conference on Thermo- and Fluid-Dynamic Processes in Diesel Engines, Birmingham, for example, the addition of hydrogen to recirculated exhaust gas has been described.
In order to ensure compliance with future limits on marine emissions there is a need to further reduce emissions, in particular the emission of nitrogen oxides.